Theoretical determination of molecular structure and conformation. IV. Electronic effects influencing the stability of methyl substituted primary ozonidesa),b)

Abstract

The conformational surfaces of three methyl substituted primary ozonides (PO), namely, 4-methyl-1,2,3-trioxolane (propene PO), cis-4,5-dimethyl-1,2,3-trioxolane (cis-2-butene PO) and the corresponding trans PO (trans-2-butene PO), are studied with restricted Hartree–Fock (RHF) theory employing a split valence [3s2p/2s] and an augmented split valence [3s2p1d/2s] basis set. An analysis of the computed RHF energies reveals that only with polarization functions in the basis set is a reliable theoretical description of the three PO’s guaranteed. In the case of propene PO and trans-2-butene PO, reasonable results are obtained by means of a rigid pseudorotor model based on the theoretically determined structures of various 1,2,3-trioxolane conformations. However, a flexible pseudorotor model is necessary in the case of cis-2-butene PO in order to achieve a realistic account of substituent–substituent interactions. The conformational surfaces of all three PO’s closely resemble that of the parent PO, 1,2,3,-trioxolane. Thus, the preferential interconversional process of a methyl substituted PO is pseudorotation hindered by barriers between 2.3 and 3.5 kcal/mole. In marked contrast to previous semiempirical results, the oxygen envelope (E) conformations are calculated to be the most stable puckered PO forms. This finding is most important with regard to the stereochemical aspects of the ozonolysis mechanism. The methyl groups either stabilize or destabilize the puckered PO ring depending on the position of the substituent. With the aid of the pseudorotation phase angle and a well-defined substituent orientation angle, reliable predictions with regard to the electronic effect of a methyl group can be made. It is shown that only an equatorially placed methyl group leads to a favorable orbital mixing between the π-type MO’s of the ring and the substituent and a resultant lowering of the two highest occupied MO’s of the PO. The puckered ring is stabilized by back-donation of electron charge to the substituent which is not possible if this is axially positioned.